1. Field of Invention
The present invention relates to point to point wireless data links.
2. Description of Related Art
FIG. 1 shows a block diagram of a prior art system 110 at a site linked to another site by a single point to point wireless data link. In operation in the transmit mode, a data port 112 is connected to a protocol engine 114 to generate the link data stream. Protocol engine 114 adds link management overhead and re-organizes the frame. A coding unit 116 adds redundancy to the data for the purpose of forward error correction. A modulator 118 translates the bits streams to quadrature amplitude modulation (QAM) symbols (mapping). A transmitter 122 up-converts the symbols to high frequency from IF/baseband frequency. In operation in the receive mode, the symbols received are down-converted by receiver 124 to IF/baseband frequency. A demodulator 120 translates the symbols into bit streams. Decoding unit 116 removes redundant data for forward error correction. Protocol engine 114 removes overhead and recovers the original frames. The data stream is outputted through data port 112. A duplexer or T/R switch 126 allows frequency division duplex (FDD) or time division duplex (TDD). In most cases an antenna 128 has a single polarized feeding network (vertical, horizontal, left hand circular, or right hand circular). A control unit 130 coordinates the link internal and external operations.
There is a fundamental limit on the capacity of a link which can be achieved for a certain bandwidth B. Capacity can be explained as the maximum number of bits that can be transmitted per second with a probability of error arbitrarily close to zero. According to Shannon""s well known capacity theorem, the error free capacity C of a noise limited link of bandwidth B is:                     C        =                              B            ·                                          log                2                            ⁡                              (                                  1                  +                  SNR                                )                                              ⁢                      bits            second                                              (        1        )            
Where SNR is the signal to noise ratio, which in turn holds all the parameters of the link budget.
In FIG. 1, the usage of a dual polarization feeding network for antenna 128, provides a doubling of the number of links 110 for the same frequency and therefore a doubling of capacity. There is an additional cost in complexity, because most of the hardware has to be doubled and an additional function called cross polar interference canceller (XPIC) has to be added. Because the two polarizations are orthogonal, the two links 110 are independent and the required transmit power for each link remains the same.
However, if it is desired to increase the capacity by more than two-fold other methods must be adopted because there are only two orthogonal polarizations. Based on Shannon""s theorem, in order to increase capacity in link(s) 10, signal power needs to be increased by a much larger amount, assuming other variables such as noise, bandwidth, antenna gain and receiver sensitivity are constant. From equation (1), it can be shown that if the capacity of link(s) 110 is doubled, assuming the other variables are constant, the required signal power must be approximately squared.
In order to provide a more reasonable increase in power for a given increase in capacity, multiple (y) independent links operating on the same frequency can be used. Independent links are links whose signals are differentiable from one another. It can be shown that the capacity of y independent links is proportional to the number of links multiplied by the capacity of one link. See for example xe2x80x9cChannel capacity of two antenna BLAST architecture, S. L. Lokya, Electronics Letters, Aug. 19, 1999, Volume 35 No. 17.
The number y can also be thought of as equaling the number of transmitting antennas. As an example if the bit rate of a system using one dual polarized transmitting                     C        =                              y            ·                                          log                2                            ⁡                              (                                  1                  +                                      SNR                    y                                                  )                                              ⁢                      bits            second                                              (        2        )            
antenna (two links) is 622 Mbits/sec, then for two dual polarized transmitting antennas (four links) the bit rate will be 1244 Mbits/sec
The challenge is therefore to create multiple links operating over the same frequency which are independent of or isolated from each other.
There has been some reporting in the prior art of the usage of the same number of transmitting and receiving antennas for multiple links when the link ends are not in line of sight of each other. For these systems to be efficient, multipath is assumed.
However, for point to point links operating with a clear line of sight between the two ends of the links, in the prior art the number of receiving antennas is larger than the number of transmission antennas in order to separate out the information belonging to each link. See for example, Advances in Spectrum Analysis and Array Processing, Volume 3, Chapter 1 by Simon Haykin, Prentice Hall PTR, May 1995, ISBN 130615404. The complexity of the processing is also high.
What is needed in the art is a system and method for multiple point to point line of sight links where the number of receiving antennas can be equal to the number of transmission antennas. What is also needed in the art is an improved beam forming algorithm using multiple statistical values calculated from a constellation or subsets thereof to allow faster convergence.
According to the present invention there is provided a system for increasing the potential communication capacity between two sites within line of sight of one another, comprising: a plurality of point to point links between the two sites, the plurality of links including at least one beam forming unit and a total of four antennas, wherein a geometrical configuration of the four antennas allows the at least one beam forming unit to substantially differentiate between signals transmitted over the plurality of links, thereby increasing the potential communication capacity compared to a system with only two of the four antennas. According to the present invention, there is also provided a method for increasing the potential communication capacity between two sites within line of sight of one another, including the steps of: providing a total of four antennas at the two sites; selecting a range of values for a function of electrical phase difference for signals to be transmitted between the antennas; and conforming a geometrical configuration of the antennas to a value within the range, wherein the range allows substantial differentiation of transmitted signals and therefore an increase in potential capacity compared to a method providing only two of the four antennas.
According to the present invention, there is further provided a method for increasing the communication capacity between two sites within line of sight of one another, including the steps of: providing two antennas at one site of the two sites, wherein the two antennas at the one site and two antennas at a second site of the two sites have a geometrical configuration which allows substantial differentiation of transmitted signals; and transmitting signals from each of the two antennas at the one site to each of the two antennas at the second site, thereby increasing the capacity compared to a method providing only one of the two antennas at the one site.
According to the present invention, there is still further provided, a method for increasing the communication capacity between two sites within line of site of one another, including the steps of: providing two antennas at one site of the two sites, wherein the two antennas at the one site and two antennas at a second site of the two sites have a geometrical configuration which allows substantial differentiation of signals; each of the two antennas at the one site receiving signals from each of the two antennas at the second site; and performing beam forming to differentiate between signals originating from each of the two antennas at the second site, thereby increasing the capacity compared to a method providing only one of the two antennas at the one site.
According to the present invention, there is still further provided a method for performing beam forming on signals which include symbols encoded by quadrature amplitude modulation, comprising the steps of: providing a first statistical value derived from a constellation which includes all possible symbols; providing at least two other statistical values derived from at least two subsets of the constellation; for a first plurality of iterations, updating filter coefficients using the first statistical value; and for a second plurality of iterations following the first plurality of iterations, updating the filter coefficients using the at least two other statistical values, wherein the updating step using the at least two other statistical values includes for each iteration the step of deciding which of the at least two other statistical values to use.
According to the present invention, there is still further provided a program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform method steps for performing beam forming on signals which include symbols encoded by quadrature amplitude modulation, comprising the steps of: providing a first statistical value derived from a constellation which includes all possible symbols; providing at least two other statistical values derived from at least two subsets of the constellation; for a first plurality of iterations, updating filter coefficients using the first statistical value; and for a second plurality of iterations following the first plurality of iterations, updating the filter coefficients using the at least two other statistical values, wherein the updating step using the at least two other statistical values includes for each iteration the step of deciding which of the at least two other statistical values to use.
According to the present invention, there is still further provided an improved computer program product comprising a computer useable medium having computer readable program code embodied therein for performing beam forming on signals which include symbols encoded by quadrature amplitude modulation, the computer program product comprising: computer readable program code for causing the computer to provide a first statistical value derived from a constellation which includes all possible symbols; computer readable program code for causing the computer to provide at least two other statistical values derived from at least two subsets of the constellation; computer readable program code for causing the computer to for a first plurality of iterations, update filter coefficients using the first statistical value; and computer readable program code for causing the computer to for a second plurality of iterations following the first plurality of iterations, update the filter coefficients using the at least two other statistical values, wherein the updating step using the at least two other statistical values includes for each iteration the step of deciding which of the at least two other statistical values to use.
According to the present invention, there is still further provided, a system for increasing the potential capacity between two sites within line of sight of one another, comprising: two antennas at a first of the two sites, wherein the two antennas at the first site, and two antennas at a second of the two sites have a geometrical configuration which allows substantial differentiation of signals transmitted by the first site and therefore increased potential capacity than a system with only one antennas at the first site.
According to the present invention, there is still further provided, a system for increasing the potential capacity between two sites within line of sight of one another, comprising: at least one beam forming unit at a first of the two sites; two antennas at the first site, wherein the two antennas at the first site and two antennas at a second of the two sites have a geometrical configuration which allows the at least one beam forming unit to substantial differentiate between signals received by the first site, thereby increasing potential capacity compared to a system with only one antenna at the first site.
According to the present invention, there is still further provided, a system for increasing the potential capacity between two sites within line of sight of one another, comprising: two antennas at a first of the two sites, wherein the two antennas at the first site, and two antennas at a second of the sites have a geometrical configuration which allows substantial differentiation of signals transmitted by the first site and therefore increased potential capacity than a system with only one antennas at the first site; the geometrical configuration being according to:       base    ⁢          xe2x80x83        ⁢    line    ⁢          xe2x80x83        ⁢    distance    ≅                    c        ·                  (                      link            ⁢                          xe2x80x83                        ⁢            distance                    )                ·                  (                      α            -            β                    )                            2        ⁢                  xe2x80x83                ⁢        π        ⁢                  xe2x80x83                ⁢        f        ⁢                  xe2x80x83                ⁢                  sin          ⁡                      (                          θ              1                        )                          ⁢                  sin          ⁡                      (                          θ              2                        )                              
wherein 2nxcfx80 less than (xcex1xe2x88x92xcex2) less than (2n+2)xcfx80, where n ∈(0,1,2, . . . )
According to the present invention, there is still further provided, a system for increasing the potential capacity between two sites within line of sight of one another, comprising: at least one beam forming unit at a first of the two sites, two antennas at the first site, wherein the two antennas at the first site and two antennas at a second of the two sites have a geometrical configuration which allows the at least one beam forming unit to substantial differentiate between signals received by the first site, thereby increasing potential capacity compared to a system with only one antenna at the first site; the geometrical configuration being according to:       base    ⁢          xe2x80x83        ⁢    line    ⁢          xe2x80x83        ⁢    distance    ≅                    c        ·                  (                      link            ⁢                          xe2x80x83                        ⁢            distance                    )                ·                  (                      α            -            β                    )                            2        ⁢                  xe2x80x83                ⁢        π        ⁢                  xe2x80x83                ⁢        f        ⁢                  xe2x80x83                ⁢                  sin          ⁡                      (                          θ              1                        )                          ⁢                  sin          ⁡                      (                          θ              2                        )                              
wherein 2nxcfx80 less than (xcex1xe2x88x92xcex2) less than (2n+2)xcfx80, where n ∈(0,1,2, . . . )
According to the present invention, there is still further provided, a method for increasing the communication capacity between two sites within line of sight of one another, including the steps of: providing two antennas at one site of the two sites, wherein the two antennas at the one site and two antennas at a second site of the two sites have a geometrical configuration which allows substantial differentiation of transmitted signals; and transmitting signals from each of the two antennas at the one site to each of the two antennas at the second site, thereby increasing the capacity compared to a method providing only one of the two antennas at the one site;
wherein the geometrical configuration is according to:       base    ⁢          xe2x80x83        ⁢    line    ⁢          xe2x80x83        ⁢    distance    ≅                    c        ·                  (                      link            ⁢                          xe2x80x83                        ⁢            distance                    )                ·                  (                      α            -            β                    )                            2        ⁢                  xe2x80x83                ⁢        π        ⁢                  xe2x80x83                ⁢        f        ⁢                  xe2x80x83                ⁢                  sin          ⁡                      (                          θ              1                        )                          ⁢                  sin          ⁡                      (                          θ              2                        )                              
xe2x80x83and 2nxcfx80 less than (xcex1xe2x88x92xcex2) less than (2n+2)xcfx80, where n ∈(0,1,2, . . . )
According to the present invention, there is still further provided, a method for increasing the communication capacity between two sites within line of site of one another, including the steps of: providing two antennas at one site of the two sites, wherein the two antennas at the one site and two antennas at a second site of the two sites have a geometrical configuration which allows substantial differentiation of signals; each of the two antennas at the one site receiving signals from each of the two antennas at the second site; and performing beam forming to differentiate between signals originating from each of the two antennas at the second site, thereby increasing the capacity compared to a method providing only one of the two antennas at the one site; wherein the geometrical configuration is according to:       base    ⁢          xe2x80x83        ⁢    line    ⁢          xe2x80x83        ⁢    distance    ≅                    c        ·                  (                      link            ⁢                          xe2x80x83                        ⁢            distance                    )                ·                  (                      α            -            β                    )                            2        ⁢                  xe2x80x83                ⁢        π        ⁢                  xe2x80x83                ⁢        f        ⁢                  xe2x80x83                ⁢                  sin          ⁡                      (                          θ              1                        )                          ⁢                  sin          ⁡                      (                          θ              2                        )                              
and 2nxcfx80 less than (xcex1xe2x88x92xcex2) less than (2n+2)xcfx80, where n ∈(0,1,2, . . . )